a good book for preparation for the first stage tests is called "Know your own IQ" by a guy called H.J. Eyshenk (I think that's his name, or something similar. Alot of the tests are similar to that which you will face when doing your NATS tests.
When I was doing the number computer test I seem to remember saying "doh" alot and still passed.
Good luck to all future ATCO's, I don't think I've met many ATCO's who actually thought that they had passed the computer tests so don't worry to much if you don't think you've passed, you could be pleasantly surprised.
Good luck